swrast: Only avoid empty _TexEnvPrograms

swrast: Only avoid empty _TexEnvPrograms

If the generated shader for _TexEnvProgram is empty, force the use of
the fixed-function code.  Otherwise, go ahead and use the shader.
This works around a mysterious issue on i915 where fixed-function
software fallbacks are not working correctly.

This isn't really the fix we want, but it works around the issue.

Signed-off-by: Ian Romanick <ian.d.romanick@intel.com>
Reviewed-by: Chad Versace <chad.versace@linux.intel.com>
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=45872
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=45876